Accuracy of cuts and drilled holes is important in knee arthroplasty, wherein installation of the implants such that the kinematics of the natural knee are duplicated as much as possible, is important to the success of the total knee replacement. To achieve this, the use of CAS systems for orthopedic operations in general, and for total knee replacement surgery in particular, is becoming increasingly more commonplace with advancements in CAS equipment that ensure improved accuracy, fail safe operation and increasing ease of use.
Known camera based CAS system employ passive and active trackable elements affixed to objects, such as surgical tools and patient bone references, in order to permit the determination of position and orientation of the objects in three-dimensional space. Preoperatively taken images or computer generated models created from preoperative patient scans, are used to provide accurate patient anatomical information to which the real-time position of the same anatomical elements can be registered or calibrated, thereby permitting subsequent tracking of the anatomical elements and display of these elements relative to the surgical tools used during the surgery.
Total knee replacement surgery requires several precise cuts to be made in the femur and tibia, such that the implant fits correctly and best replicates the geometry of a natural healthy knee. To perform these steps, in both conventional and CAS total knee replacement, it is well know to use a guide block which provides a drill and/or cutting guide to assist the surgeon to perform the steps required to prepare the femur and tibia for receiving the implant.
In order to best understand the improvement the present invention provides over such guide blocks of the prior art, it is necessary to understand the steps performed during a typical total knee replacement surgery to prepare the bones for receiving the implants.
The typical method steps used to prepare the femur for a knee replacement implant, outlined below as an example, generally include: fastening a guide block on the femur, generally located by an intramedullary pin or screw inserted into the distal end of the femur and locating the guide block in the desired position; aligning a distal cutting guide, whether being integral with the guide block or a separate element fastenable thereto, in a predetermined location relative to the guide block reference position and inserting locating pins through the distal cutting guide and into the femoral condyles to fasten the cutting guide in place on the anterior surface of the distal end of the femur; removing the distally mounted guide block, leaving the distal cutting guide pinned to the anterior surface of the femur; making the distal cut to resect the predetermined amount of bone from the distal end of the condyles; positioning the guide block freely on the newly cut distal surface of the femur and ensuring that the resection level for the anterior cut, the anterior-posterior adjustment for implant sizing, the rotational alignment and medial-lateral position of the positioning block are all correct before fixing the guide block in place with pins; removing the positioning guide block, putting the peg hole drill guide block onto the pins, and drilling the implant peg holes; and using these peg holes to install an anterior-posterior cutting block which is then used to perform the anterior cut, and subsequently to install an appropriately sized chamfer cutting block which is then used to make the anterior-posterior chamfer and notch cuts.
The steps required to prepare the tibia are less involved. Generally, they include: aligning the mechanical axis of the tibia; obtaining proper rotational alignment of the guide block, and fastening it in place to the anterior surface of the proximal end of the tibia; adjusting the guide block to ensure the desired posterior slope and level of tibial resection are provided; inserting location pins using the guide block; removing the guide block and replacing it with a tibial resection cutting guide that is retained in place with the location pins; and resecting the chosen amount of tibial bone.
The above surgical procedures remain generally similar whether traditional or computer assisted surgery is being performed. As such, the use of a cutting/drill positioning block having a position identifying member fastened thereto and trackable by a camera based CAS system, for example, is known for use in total knee replacement surgery. However, while such tracked femoral positioning guide blocks provide significant advantages over traditional non-CAS instruments, there nevertheless remains room for improvements to the current guide blocks used in total knee replacement surgery, whether being a guide block for use with an image guided CAS system or traditional non-computer aided surgery, in order to further simplify surgical procedures and to enhance accuracy.
As discussed above, known total knee replacement procedures include creating a distal on the femur in order to resect enough bone to permit the installation of the femoral implant. In conventional, or non computer assisted, total knee replacement surgery a distal cutting block is positioned and aligned by the surgeon and pinned in place on the anterior surface of the femur such that the cutting slot is aligned in the correct location for the distal cut. In CAS total knee replacement, it is also known to use a distal pin drill guide to accurately create the pin holes into which locating pins are inserted and employed to fix the distal cutting guide, either integrally formed with the distal pin drill guide or being a separate element, in the correct location to make the distal cut in the femur. Generally, the distal drill/cutting guide member comprises part of an assembly including an anterior guiding platform, that is fixed relative to the femur and on which the drill/cutting guide is displaceable by a selected, measurable amount to locate the drill/cutting guide in a desired position relative to the anterior guiding platform and therefore relative to the distal end of the femur. A tracked guide block is often intramedullarly fastened to the femur, and the anterior guiding platform can then be engaged thereto. Depending on the type of implant being used, and once aligned with the most distal femoral condyle, the drill/cutting guide can then be proximally displaced on the fixed anterior guiding platform by a selected amount corresponding to the amount of bone to be resected.
A captive spring loaded plunger, located within the distal drill/cutting guide, is known to comprise a pointed pawl portion, which engages a series of notches located on the anterior guiding platform to fix the distal drill/cutting guide in place thereon. Demarcations on the guiding platform indicate resection distance, and the spring loaded plunger can be depressed to release the pawl from the notches and consequently permit movement of the drill/cutting guide along the guiding platform. As the notches are formed such that the distance between each notch accurately corresponds to a single unit of distance, for example 2 mm, a precise resection distance can be achieved by depressing the spring-loaded plunger and sliding the guide on the fixed platform the required number of notches. Demarcations on the platform provide a visual indication of the position of the guide block position.
This spring loaded mechanism is effective to permit displacement of the drill/cutting guide when required and to fix the guide in place when the guide is correctly aligned by releasing the outwardly biased plunger. However, such currently known mechanisms generally use a blind hole which receives a helical spring and the plunger therein, the plunger being permanently retained within the guide. This often causes sterilization problems, as cleaning the spring and inside bore of the mechanism becomes difficult because the captive plunger can not be removed. As a result, bacteria can build up inside the bore, and can not be easily cleaned out and sterilized. This becomes a major problem as cleanliness is paramount in surgical environments.
As CAS systems permit improved visualization of the positioning block relative to the bone elements of the femur and projected reference block axes superimposed relative to those of the bone element, fewer fixed anatomical reference surfaces need to be used in conjunction with tracked CAS positioning reference blocks. However, to best permit temporary fixation block in a determined position, the reference block requires controllable adjustment of several degrees of freedom. While certain flexibility is provided by total knee replacement positioning guide blocks of the prior art, there nevertheless remains a need for a more universal positioning block permitting additional controllable flexibility of movement, and being adapted for use with a CAS system.